By way of background, many transmissions contain multiple mechanical gears which apply different degrees of speed reduction or increase to the RPM of the engine coupled to the transmission. Conventionally, the forward gears are numbered sequentially, e.g. F1, F2, etc., in descending gear ratio order so that, for example, F1 provides the highest gear ratio and greatest speed reduction from engine speed for the greatest output torque. Off-road work vehicle transmissions also often have multiple reverse mechanical gears. Again, by convention, they are numbered sequentially, e.g. R1, R2, etc., in descending gear ratio order.
Off road work vehicles, such as agricultural or construction tractors or crawlers, are frequently used in shuttle operations in which the vehicle is driven forward and backward repeatedly and the forward and reverse speeds are related. The desired reverse speed may be the same as or slower or faster than the forward speed used. In some electronically controlled automatic transmissions, this forward/reverse relationship can even be programmed. The forward and reverse gears can be preselected or the reverse gear can be selected to be displaced upward or downward from the forward gear used such that the reverse speed may be one or more reverse gears higher than that of the selected forward gear, or the same as the selected forward gear, or one or more gears lower than that of the selected forward gear. The operation described above is ideal for transmissions that have gear paths that provide reverse gear ratios equal to equally spaced forward gear ratios for each gear ratio in the range of normal shuttling speeds used.
A shuttle shifting configuration in a power shift transmission enables the off-road work vehicle to “shuttle” back and forth (i.e., in forward and in reverse) when doing work or other activities without the operator actually having to identify and find the reverse gear to be used in the shuttle. This relationship between a forward gear and a reverse gear can be preset. A lever can be provided to shuttle shift from forward to reverse and vice-versa, based upon the present relationship, simplifying the procedure for the operator of the off road work vehicle. Some transmissions utilized in shuttle-shifting vehicles have reverse mechanical gears with reverse gear ratios which correspond to the gear ratios of consecutive forward mechanical gears. For example, in a transmission with twelve forward mechanical gears and five reverse mechanical gears, the five reverse mechanical gears will have gear ratios that correspond to five consecutive forward mechanical gears, such as one to five, three to 7, eight to twelve, etc. The shuttle-shifting technique, however, would not work well with a power shift transmission that does not have reverse gear ratios corresponding to the forward gear ratios or where some reverse gear ratios are missing, and the present invention addresses that problem.
U.S. Pat. No. 5,036,718 issued on Aug. 6, 1991, is titled “Shuttle Shift Transmissions,” and its subject matter is incorporated by reference herein in its entirety. U.S. Pat. No. 5,012,690 issued on May 7, 1991, is titled “Programmable Shuttle Shifting in Transmissions,” and its subject matter is incorporated by reference herein in its entirety. U.S. Pat. No. 5,526,261 issued on Jun. 11, 1996, is titled “Pseudo-Gear Transmission Control System and Method,” and its subject matter is incorporated by reference herein in its entirety. U.S. Pat. No. 6,845,682 issued on Jan. 25, 2005, is titled “Smoothly Shifting Multispeed Transmission,” and its subject matter is incorporated by reference herein in its entirety.